The present invention relates to a plastic lens assembly for use in a copying machine, and more particularly to a completely symmetrical lens assembly composed of plastic lenses.
There have been recent demands for smaller, lighter, and less costly lenses. To meet such demands, especially for lighter and more inexpensive lenses, a larger number of lenses are now made of plastics. Plastic are advantageous for use in lens in that plastic lens are of a small specific gravities and are inexpensive. However, the lenses of plastic materials are relatively unstable at varying temperature with the radius of curvature, the thickness, and the refractive index of such a plastic lens being subject to a large temperature-dependent changes. Furthermore, certain plastic material, such as polymethyl methylacrylate, are as susceptible to humidity as it is to temperature. Lenses made of such a plastic material are therefore problematic since their focal length tends to change as a function of either temperature and/or humidity. For these reasons, prior plastic lenses have been used primarily in limited applications such as optical systems with automatic focusing capability, e.g., an objective lens designed for focusing a laser beam onto a compact disc, and optical systems with relatively large tolerances, e.g., a viewfinder optical system.
Nevertheless, plastic materials are finding use in lens systems such as optical systems for copying machines where they are subject to heat and the conjugate length of the lens systems is kept constant. For example, Japanese Laid-Open Patent Publication No. 60(1985)-67915 discloses a five-element lens system, the four lenses of which are made of plastics. The disclosed lens system is however impractical because no consideration is given to humidity-dependent changes in optical properties, though it is temperature-compensated.